The present invention relates to optical recording using more than one laser beam to record a track.
Optical recording consists of the controlled marking of a media surface using a focused and modulated laser beam. The laser beam heats regions of a recording layer to a media-specific critical temperature. In the regions so heated, the medium undergoes a visible change that may be subsequently detected under lower power illumination.
FIG. 1 illustrates a prior art optical recording process. The output of a recording laser 10 is modulated by data channel electronics 12. A focusing lens 14 focuses the modulated laser beam to a small recording spot 16 on the surface of an optical recording medium 20. The medium is scanned relative to the focused spot in a scanning velocity 18. The recording medium absorbs laser beam radiation and is heated locally in the vicinity of the focused spot. Visible recorded marks 22 form on the media surface at locations that have been heated above the critical temperature.
If the recording medium lacks sufficient sensitivity, or if the recording laser lacks sufficient power, the recording medium will nowhere reach the critical temperature and no marks will form. In FIG. 2 prior art, data channel electronics 12 modulate a laser beam from a low power source recording laser 10xe2x80x2. The laser beam is focused by lens 14 to form a recording spot 16xe2x80x2 of reduced intensity on the surface of the recording medium 20. With reduced intensity, the spot heats the media surface to a lesser degree, resulting in recorded marks 24 that are smaller than the recording spot and irregular in size.
For many applications and system designs, the rate of optical recording is limited by the available laser power. It is well known that less recording power is required when the media is already hot. Thus, the effective media sensitivity may be increased by preheating (see U.S. Pat. Nos. 4,383,261 and 4,530,080).
One prior art method of preheating is shown in FIG. 3. A low power beam from recording laser 10xe2x80x2 is modulated by data channel electronics 12. The laser beam is focused by a lens 14 to form a focused spot 16xe2x80x2 with reduced intensity at the surface of the optical recording medium 20. A heating element 28 positioned upstream from the focused spot heats the media surface to a temperature lower than a critical recording temperature. The heated zone 30 of the medium extends at least to the focused recording spot. Because the laser heating is added to the preheating of the medium, the laser pulses create larger recorded marks 26. However, preheating also causes the mark shapes to be irregular in shape. The shape irregularity arises because local variations in media characteristics or recording system parameters are exaggerated when the thermal gradients across the medium are small during the recording event.
An alternative prior art method of preheating for optical recording is to provide a second focused optical spot that heats the medium immediately upstream from the recording spot, as in FIG. 4. A low power laser beam from recording laser 10xe2x80x2 modulated by data channel electronics 12 is focused by the focusing lens 14 onto the recording medium 20. The beam from a second, unmodulated preheat laser 32 also passes through the focusing lens, creating a preheat spot 34 located immediately upstream from the recording spot. The width of the preheat spot is substantially the same as the recording spot. The preheat spot heats the medium to a temperature less than a critical marking temperature in a heated media zone 30xe2x80x2 that extends to the recording spot 16xe2x80x2. In order to assure that the preheated media zone extends to the recording spot, the preheat spot must be closer to the recording spot than the scan velocity 18 multiplied by about 2 microseconds. The pulses of the recording laser create recorded marks 26 that have increased size because of the preheating. The irregularity of the mark shapes is reduced because the mark width is confined to the narrow preheated zone. However, preheating still causes some variability in mark length.
Unintended variation in recorded mark length and width degrades signal quality, inducing readout errors and limiting the storage capacity and channel performance of the recording medium.
It is an object of this invention to increase the effective media sensitivity for optical recording without substantially increasing the length irregularity of the recorded marks.
This object is achieved by a method for optical data recording, comprising:
a) focusing a preheat laser beam and a recording laser beam onto an optical recording medium, forming a preheat focused spot and a recording focused spot having nearly equal sizes;
b) providing relative scanning motion between the optical recording medium and the recording focused spot in a direction parallel to the displacement from the preheat focused spot to the recording focused spot at a velocity sufficient to prevent thermal diffusion from the optical recording medium during the transit of the medium from the preheat focused spot to the recording focused spot;
c) modulating the preheat laser beam according to a modulated data signal, thereby creating preheated zones on the media surface with temperatures less than a threshold for complete mark formation;
d) modulating the recording laser beam according to the modulated data signal, with a time delay equal to the separation between the preheat focused spot and the recording focused spot divided by the scanning velocity; and
e) causing the recording laser beam to illuminate the recording focused spot with sufficient power to raise the temperatures of each preheated zone above the threshold for complete mark formation, creating a track of permanent recorded marks.
It is an advantage of the present invention that effective high speed optical recording can be achieved using preheat laser beams without the need for high power laser sources.
It is another advantage that by practicing the present invention, high fidelity recording can be achieved.
It is a further advantage of the present invention that VSCEL arrays may be utilized for multitrack optical recording.